1. Field of the Invention
The present invention relates to a control apparatus for an automatic transmission of a vehicle such as a motor vehicle in which the transmission has a plurality of hydraulic engaging elements for speed changing.
2. Description of Related Art
As this kind of control apparatus, there has conventionally been known a control apparatus which comprises a plurality of speed-change control valves corresponding to a plurality of hydraulic engaging elements and a plurality of speed-change solenoid valves corresponding to these speed-change control valves. The hydraulic oil pressure in each of the plurality of hydraulic engaging elements is adjusted depending on a signal pressure from a corresponding speed-change solenoid valve with a line pressure from a hydraulic oil pressure source serving as a base pressure. In this apparatus, the hydraulic oil pressure in each of the hydraulic engaging elements can be directly controlled by a controller which is made up of a vehicle-mounted computer through each of the speed-change solenoid valves. Therefore, the apparatus is advantageous in reducing the speed change shocks.
Further, there is also known an apparatus which is provided with a pressure regulating means for varying the line pressure, wherein the line pressure is decreased at the time of cruising of the vehicle so that the driving load of the hydraulic oil pressure source can be reduced, thereby improving the specific fuel consumption.
In the above-described apparatus in which the hydraulic oil pressures in the hydraulic engaging elements are controlled depending on the signal pressures from the speed-change solenoid valves, the deterioration in the controllability of the speed-change solenoid valves by the clogging or inclusion of foreign matter in the speed-change solenoid valves will be a big problem in commercializing the product.
Foreign matter which has clogged or has found its way into a speed-change solenoid valve can be removed by opening the solenoid valve. Thus, there is known an apparatus in which cleaning work is performed by intermittently opening the speed-change solenoid valve in a neutral range or in a parking range to thereby remove the foreign matter.
In the above-described conventional apparatuses, cleaning work is not performed while the vehicle is running in a running range. Therefore, there is a problem in improving the toughness against (or the ability to successfully cope with) the clogging or inclusion of foreign matter in the speed-change solenoid valve.
In view of the above-described points, the present invention has an object of providing a control apparatus for an automatic transmission of a vehicle in which the cleaning work to remove foreign matter can be performed during running of the vehicle without affecting the hydraulic oil pressure in the hydraulic engaging elements, whereby the toughness against the inclusion of foreign matter is improved.